MRI is a relatively recent diagnostic imaging modality. In addition to providing contrast resolution and spatial resolution, MRI offers tissue information and data about metabolic processes in the body. MRI provides multiple parameters for obtaining image information, some of which carry a great deal of information, and some of which can be manipulated to obtain a tissue diagnostic image. These parameters are: (1) pro,on density, (2) tissue relaxation parameters T1 (spin lattice) and T2 (spin-spin) and (3) motion, which provides information about blood flow.
Thus, MRI has been developed as an imaging modality used to obtain images of anatomical features of human patients as well as some aspects of the functional activity of biological tissue. The images have medical diagnostic value in determining the state of health of the tissue examined. To obtain images, typically, the patient is aligned to place the portion of the anatomy to be examined in the imaging volume of a MRI apparatus. The apparatus typically comprises a primary magnet for supplying a constant magnetic field (B.sub.o) which by convention is along the z-axis and is substantially homogeneous over the imaging volume and secondary magnets that can provide linear magnetic field gradients along each of three principal Cartesian axes in space (generally x, y, and z, or x.sub.1, x.sub.2 and x.sub.3, respectively). A magnetic field gradient (.delta.B.sub.z /.delta.x.sub.i) refers to the variation of the field along the direction parallel to B.sub.o with respect to each of the three principal Cartesian axes, x.sub.i. The apparatus also comprises one or more RF (radiofrequency) coils which provide excitation and detection of the NMR signal.
A number of methods have been proposed and used to spatially encode the NMR signal in order to perform MRI. These techniques can be classified as follows.
1. Fourier Encoding. Perhaps the most basic of methods to obtain one dimensional spatial information is to apply a magnetic field gradient to excited nuclear spins. The Larmor precession frequency is then linearly related to position in the gradient. The observed signal, when Fourier transformed, gives a projection of the object in the direction of the gradient. PA1 2. Field Profiling Techniques. These methods create a small region where the field is homogeneous, "sweet spot", with the field rapidly decreasing at the edges such that the NMR signal within a limited bandwidth (i.e. limited frequency range) is limited to the small homogeneous region. This technique is also known as topical magnetic resonance TMR. To image the object, it is moved so that different regions of the object are located in the "sweet spot" and hence imaged in a sequential way. See, for example, U.S. Pat. No. 3,932,805, which is hereby incorporated by reference. PA1 3. Sensitive Point Techniques. These techniques create a "sweet spot" by oscillating the magnetic field gradients as a function of time. In the sensitive point method, all three gradients are modulated such that there is one point where the field from each gradient is zero and does not change as a function of time. The NMR signal, when averaged as a function of time, is finite only in a small region surrounding this point. This sensitive point is then electronically moved to different regions of the object to be imaged in order to obtain the NMR signal from different spatial regions. See, for example, U.S. Pat. No. 4,015,192, which is hereby incorporated by reference. PA1 4. Sensitive Line Techniques. This method is similar to the sensitive point technique except that only two gradients are modulated such that a "null line" is defined. The signal obtained is from a small cylindrical region whose axis is defined by this null line. The line is then scanned across the plane to be imaged. Spatial information along the line is resolved by having a constant linear magnetic field gradient applied in the direction of the line such that the NMR Larmor precession frequency depends linearly on position along the line. Hence, simple Fourier transformation of the NMR signal resolves position along the line. PA1 5. Selective Techniques Which Use Magnetic Field Gradients. These include sequential application of orthogonal linear gradients and bandwidth limited excitation pulses to first excite a plane, then a line in the plane, and "reading out" the NMR signal in the presence of a third orthogonal linear gradient. The observed NMR signal is Fourier transformed to obtain spatial information along the final selected "line". See, for example, U.S. Pat. No. 4,021,726, which is hereby incorporated by reference. PA1 6. Rotating Frame Zeugmatography. In this method, a radio frequency field which has a spatial gradient is applied to the object to be imaged. By varying the amount of amplitude or duration of RF energy applied to the coil, material in the object being imaged is excited by varying amounts. As the amount of energy applied to the RF gradient coil increases, the magnetization components at a given point in the object undergo a sinusoidal variation. The larger the RF field is in the object, the faster the sinusoidal variation. Hence, the signal, which is proportional to the transverse magnetization, is "phase encoded". A Fourier transform of multiply obtained sets of data (i.e. each with a different amplitude of excitation of The RF coil) provides spatial discrimination along The gradient of the RF field. (See, D. I. Hoult, Journal of Magnetic Resonance, 33, 183-197 (1979))which is hereby incorporated by reference.
An extension of this method to more than one dimension is often referred to as the SPIN WARP METHOD. Different lines of k-space (i.e. The multidimensionai Fourier transform of image space) are acquired on different excitations of the nuclei. K-space is a multidimensional space in which The data is acquired. For example, consider the two dimensional case, say k.sub.x and K.sub.y. K.sub.x is defined as the phase per unit position in x and k.sub.y is the phase per unit position in y. During each excitation, a linear readout gradient is always applied during data acquisition to spatially discriminate the NMR signal along its direction (say x) and Thus acquire different k.sub.x data. In subsequent excitations an incremental phase shift in the orthogonal direction y is applied prior To the data acquisition in order to change the value of k.sub.y for the next k.sub.x line. A two dimensional Fourier transfer (2DFT) is performed on the data to give the resultant image. See, Edelstein, W. A., Hutchison, J. M. S., Johnson, G., Redpath, T. W., Phys. Med. Biol, 25:751, 1980, which is hereby incorporated by reference.
Imaging data for construction of MRI images can be collected using one of many available techniques such as, for example, those described above. Typically, such techniques use a pulse sequence which generates a plurality of sequentially implemented views. Each view comprises a field gradient pulse to encode spatial information into the NMR signal which is collected during that view. Each view may be prepared by RF excitation pulses and/or field gradient pulses prior to the collection of that view. All currently known MRI imaging techniques employ views which by themselves are one dimensional. Therefore, to obtain multidimensional information, requires the acquisition of many views. To go from one view to another in conventional techniques requires modulation of the magnetic field gradient pulses between views. The NMR signal may be a free induction decay (FID) or a spin echo signal (SE). In most of the currently adopted techniques for MRI, the signal is received in the form of a spin echo. The encoding field gradient which is present during the detection of the SE is denoted as the view or readout gradient. Two or more dimensional images are obtained by acquiring multiple views with each view spatially encoded in a different manner. Well known techniques are then applied to the multiple views in order to reconstruct an image of the tissue. The fastest MRI method available today is known as "Echo Planar Imaging" (EPI). EPI is capable of providing the data for a single planar image in approximately 100 milliseconds (ms). Multiple planar images can be collected and later combined to provide a three dimensional image. In another implementation, an additional phase encoding linear field gradient may be added to provide spatial encoding along a third spatial direction. The image is then reconstructed using three dimensional Fourier transform (3DFT) methods to provide a 3D image.
EPI is fast because it acquires all of the different phase encoded views required for a 2D image in one excitation of the nuclei. To achieve this, EPI methods require that the encoding field gradients be switched very. quickly, on the order of hundreds of microseconds (.mu.s). The speed of EPI is also dependent on the strength of the encoding fields, and further decreases in imaging time can be accomplished by using larger field gradients. The switching of large field gradients requires expensive and sophisticated amplifier technology. In addition, transitory fields induce eddy currents in the surrounding metallic structures as well as in the patient, which causes image distortion. Typically, manufacturers minimize the eddy currents in the surrounding metallic structures by employing what is known as actively shielded gradients. An actively shielded gradient coil see is carefully designed to minimize the magnetic flux extending beyond the gradient coil structure into the surrounding metallic structures of the magnet. Consequently, eddy currents are substantially reduced which allows the implementation of EPI methods. Eddy currents induced in the patient cannot be minimized except by limits imposed on the switching times of the gradient fields. Hence, the speed of EPI techniques is then limited by the time the technology permits in switching the readout and phase encoding gradients to go from one view to another.
Rapid imaging techniques have found clinical use in imaging rapidly moving structures such as The heart, in following bolus injections of contrast agents (for example to measure tissue perfusion) as well as decreasing the patient examination time. New imaging techniques are desirable for further advances in MRI.